The present invention relates generally to the field of marine geophysical surveying. More particularly, in one or more embodiments, this invention relates to towing methods and systems for controlling spread and/or depth in a geophysical survey.
Certain types of marine geophysical surveying, such as seismic or electromagnetic surveying, include towing an energy source at a selected depth in a body of water. One or more geophysical sensor streamers also may be towed in the water at selected depths. The streamers are essentially long cables having geophysical sensors disposed thereon at spaced apart locations. Actuation of the energy source emits an energy field into the body of water. The energy field interacts with the rock formations below the water bottom. Energy that interacts with interfaces, generally at the boundaries between layers of rock formations, is returned toward the surface and is detected by sensors on the one or more streamers. The detected energy is used to infer certain properties of the subsurface rock, such a structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
Current electromagnetic survey techniques are generally based on a two-dimensional arrangement with a survey vessel towing a single streamer. As the streamer is pulled through the water, one or more hydrodynamic depressors can be used to pull the streamer down to a pre-selected depth. The length of the lead-in cable interconnecting the streamer with the survey vessel can be adjusted to regulate depth of the streamer. More line depth adjustments can be made with commercially available depth control devices cooperatively engaged with the streamer.
For electromagnetic surveying, it can be important that a streamer is maintained as close as possible to a selected depth profile in the water. For example, it may be important to increase the towing depth with an optimum depth being as close as possible to the seafloor while keeping the streamer as level as possible. This towing arrangement should reduce noise originating from towing the streamer through the water. Another important issue in electromagnetic surveying is cross-line sensitivity. In general, cross-line sensitivity is the distance in the horizontal plane perpendicular to the streamer direction of travel where the sensitivity drops below a detectable limit. In seismic surveying, cross-line sensitivity has been addressed by use of a three-dimensional survey arrangement in which multiple streamers are towed at selected lateral distances from one another. Spreading devices are used in seismic surveying to achieve the desired lateral spread between the streamers, thus improving the cross-line sensitivity of the seismic survey. However, the streamers in the seismic surveys are typically towed at shallow depths (e.g., <20 m), which would result in low sensitivity due to streamer distance from the seafloor if used in an electromagnetic survey.
Accordingly, there is a need for improved methods and systems for controlling depth and spread in geophysical surveys to, for example, increase cross-line sensitivity while keeping the streamer as close to the seafloor as possible.